Semiconductor device and fabrication method thereof

ABSTRACT

Regions  106  which can be regarded as being monocrystalline are formed locally by irradiating with laser light, and at least the channel-forming region  112  is constructed using these regions. With thin-film transistors which have such a construction it is possible to obtain characteristics which are similar to those which employ monocrystals. Further, by connecting in parallel a plurality of such thin-film transistors it is possible to obtain characteristics which are effectively equivalent to those of a monocrystalline thin-film transistor in which the channel width has been increased.

BACKGROUND OF THE INVENTION DESCRIPTION OF THE RELATED ART

In recent years research has been conducted into techniques for formingtransistors which employ thin-film semiconductors on a glass or quartzsubstrate (referred to as thin-film transistors). In particular,techniques employing amorphous silicon as the thin-film semiconductorhave been put to practical use, for use in-active matrix-type liquidcrystal display devices and the like.

However, thin-film transistors which employ amorphous silicon have theproblem that their characteristics are poor. For example, if animprovement is required in the display function of an active matrix-typeliquid crystal display device, the characteristics of thin-filmtransistors which employ amorphous silicon are too poor to achieve this.

Further, techniques are known for constructing thin-film transistorsusing crystalline silicon films in which amorphous silicon films havebeen crystallized. These techniques involve transforming an amorphoussilicon film into a crystalline silicon film by performing heattreatment or irradiation with laser light after formation of theamorphous silicon film. Crystalline silicon films obtained bycrystallizing amorphous silicon films generally have a multicrystallineconstruction or a microcrystalline construction.

By constructing thin-film transistors using crystalline silicon films itis possible to obtain much better characteristics than if amorphoussilicon films are used. For example, considering the mobility, which isone index with which to evaluate the characteristics of thin-filmtransistors, with thin-film transistors employing amorphous siliconfilms the mobility is 1 cm²/Vs or less, but with thin-film transistorsemploying crystalline silicon films, a value of approximately 100 cm²/Vscan be achieved.

However, crystalline silicon films which are obtained by crystallizingamorphous silicon films have a multicrystalline construction, and thereare a number of problems which result from crystal grain boundaries. Forexample, since some carriers migrate via crystal grain boundaries, thereis the problem that voltage resistance is greatly limited. There is thefurther problem that under high speed operation, for example, variationand degradation of the characteristics is liable to occur. There isfurthermore the problem that since some carriers migrate via crystalgrain boundaries, there is a large leak current when the thin-filmtransistor is off.

Further, in order to construct active matrix-type liquid crystal displaydevices in a more integrated fashion it is desirable to form not onlythe pixel region but also the peripheral circuitry on a single glasssubstrate. In such cases the thin-film transistors which are arranged inthe peripheral circuitry must be able to handle large currents in orderto drive the many thousands of pixel transistors which are arranged inmatrix form.

In order to obtain thin-film transistors which can handle large currentsit is necessary to adopt a construction which has a wide channel.However, with thin-film transistors which employ polycrystalline siliconfilms or microcrystalline silicon films there is a problem that thiscannot be realized even if the channel is widened, due to the problem ofvoltage resistance. There is the further problem that variations in thethreshold voltage and the like are large, and they are therefore notpractical.

SUMMARY OF THE INVENTION

The invention disclosed in the present specification aims to provide athin-film transistor which is not affected by crystal grain boundaries.

Further, another aim of the invention disclosed in the present inventionis to provide a thin-film transistor which has a large voltageresistance and with which it is possible to handle large currents.

Further, yet another object of the invention disclosed in the presentspecification is to provide a thin-film transistor with which there isno degradation or variation in the characteristics.

One invention disclosed in the present specification is a semiconductordevice which employs a thin-film silicon semiconductor which is formedon a substrate which has an insulating surface, wherein

-   -   the abovementioned thin-film silicon semiconductor has a region        which can be regarded as being effectively monocrystalline,    -   the abovementioned region constitutes at least part of an active        layer,    -   and the abovementioned region contains carbon and nitrogen atoms        at a concentration of between 1×10¹⁶ cm⁻³ and 5×10¹⁸ cm⁻³,        oxygen atoms at a concentration of between 1×10¹⁷ cm⁻³ and        5×10¹⁹ cm⁻³, and hydrogen atoms, which neutralize the unpaired        bonds in the silicon, at a concentration of between 1×10¹⁷ cm⁻³        and 5×10²⁰ cm⁻³.

With the abovementioned construction, a region which can be regarded asbeing effectively monocrystalline refers to a thin-film siliconsemiconductor region which has a crystalline construction which is takento be equal to the crystallinity of a monocrystalline silicon wafer.Specifically, a region which is regarded as being effectivelymonocrystalline is defined as being a region in which the Raman spectrumintensity ratio in comparison with the Raman spectrum formonocrystalline silicon is at least 0.8, the ratio of the full-widths athalf magnitude (relative values) is 2 or less, and at the same timethere are effectively no crystal grain boundaries in said region.

Such a region which can be regarded as being effectively monocrystallinecan be obtained using an amorphous silicon film as the starting film,and heating it or irradiating it with laser light. In particular, byintroducing metal elements which promote crystallization of the silicon,it is possible to obtain the abovementioned region which can be regardedas being effectively monocrystalline, relatively easily over a largearea.

One or more elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt,Cu, Zn, Ag and Au can be used as the metal elements which promotecrystallization of the silicon. These elements have the property thatthey penetrate into silicon, and they disperse within the silicon filmwhen heat treatment or irradiation with laser light is effected. Fromamong the abovementioned elements, Ni (nickel) is an element with whichit is possible to obtain particularly striking effects.

It is important for the abovementioned metal elements to be contained inthe final silicon film after completion of crystallization at aconcentration of between 1×10¹⁶ and 5×10¹⁹ cm⁻³. If the concentration ofthe metal elements is less than 1×10¹⁶ cm⁻³ then it is not possible toobtain crystallization-promoting effects, and if the concentration ismore than 5×10¹⁹ cm⁻³ then the quality of the semiconductor is impaired.

The following methods can be used to form the region which can beregarded as being effectively monocrystalline within the silicon thinfilm. Firstly, an amorphous silicon film is formed on a glass substrateor a quartz substrate, after which a film containing nickel is formed onthe surface of the amorphous silicon film. The film containing nickelmay be one in which an extremely thin nickel film is formed by thesputtering method, for example, or a method may be adopted whereby thenickel element is disposed in contact with the surface of the amorphoussilicon film by applying a solution containing nickel onto the surfaceof the amorphous silicon film.

Having introduced the nickel element into the amorphous silicon film theamorphous silicon film is crystallized by being heat treated. This heattreatment may be performed at a temperature of 600° C. or less due tothe action of the nickel element. If a glass substrate is used as thesubstrate then it is preferable for the temperature of the heattreatment to be as low as possible, but in consideration of theefficiency of the crystallization process it is advantageous for thetemperature to be 500° C. or more, preferably 550° C. or more. It shouldbe noted that when a quartz substrate is used for the substrate, theheat treatment may be performed at a temperature of 800° C. or more, andit is possible to obtain a crystalline silicon film in a short time. Thecrystalline silicon film obtained by this process has a multicrystallineor microcrystalline form, and there are crystal grain boundaries withinthe film.

Then, by irradiating the sample with laser light, with the sample heatedto a temperature of 450° C. or more, crystallization is promotedlocally. By this process it is possible to form a region which can beregarded as being monocrystalline. When the laser light is beingirradiated, it is important to heat the sample or the irradiated surfaceto a temperature of 450° C. or more. The heating temperature ispreferably between 450° C. and 750° C., and in particular, when a glasssubstrate is being used as the substrate, between 450° C. and 600° C.

Further, another method which can be cited as a method for forming theregion which can be considered to be monocrystalline is one in which anamorphous silicon film is formed, a metal element which promotescrystallization is introduced, and the region which can be considered tobe monocrystalline is formed by irradiating with laser light withoutperforming heat treatment. In this case also it is important to heat thesample to between 450° C. and 750° C. during the irradiation with laserlight, and in particular if a glass substrate is being used for thesubstrate, to heat it to a temperature of between 450° C. and 600° C.

The significance of heating the sample during irradiation with laserlight will now be explained. FIG. 4 shows the Raman spectrum intensitywhen laser light is irradiated onto an amorphous silicon film in which abase silicon oxide film has been formed on a glass substrate, anamorphous silicon film has been formed thereon and nickel element hasbeen introduced onto the surface thereof. Further, each plotted pointindicates the temperature to which the sample was heated when the laserlight was irradiated.

The Raman intensities shown in FIG. 4 are relative values showing theratio (I/I₀) between the Raman spectrum intensity I₀ of amonocrystalline silicon wafer and the Raman spectrum intensity I of thesample. The Raman spectrum intensity is defined as being the maximumvalue of the Raman spectrum intensity, as shown in FIG. 7. In generalthere are no crystalline constructions which exceed a monocrystallinesilicon wafer, and therefore the maximum value of the Raman intensityshown on the vertical axis in FIG. 4 is 1. It can be seen that as thevalue of the Raman intensity approaches 1, the construction approaches amonocrystalline construction.

FIG. 5 shows plots of the relationship between the full-width at halfmagnitude for the Raman spectrum and the energy density of the laserlight irradiation for samples heated to different temperatures. Thefull-width at half magnitude shown on the vertical axis is a parameterindicating the ratio (W/W₀) between the width W₀ of the spectrum at aposition at half of the Raman spectrum intensity for a monocrystallinesilicon wafer and the width W of the spectrum at a position at half ofthe Raman spectrum intensity which was actually obtained for the sample.W and W₀ are defined as the width of the spectrum at a position of halfof the Raman spectrum intensity, as shown in FIG. 7. In general, anarrow, sharp Raman spectrum means that the crystallinity is excellent.Consequently, in general the width of the Raman spectrum formonocrystalline silicon is the thinnest and the sharpest. It should benoted that the samples which were used were the same as those for whichthe data shown in FIG. 4 was obtained.

Thus the full-width at half magnitude shown in FIG. 5 is generally avalue of 1 or more. It can further be seen that as the value approaches1, the construction approaches a monocrystalline construction. As can beseen from FIG. 5, it is possible to obtain a crystallinity whichapproaches that of a monocrystal if the temperature to which the sampleis heated during irradiation with laser light is increased. It canfurther be seen that the effects due to heating the sample becomesaturated at about 500° C. It can be concluded from FIG. 5 that in orderto obtain stable crystallinity which approaches that of a monocrystal,heating to 400° C. is not reliable, and it is therefore preferable toheat to 450° C. or more in order to provide some leeway.

In the opinion of the present inventors, a region can be regarded asbeing monocrystalline if the Raman intensity shown in FIG. 4 is 0.8 ormore, the full-width at half magnitude of the Raman spectrum shown inFIG. 5 is 2.0 or less, and there are effectively no crystal grainboundaries within the region.

The region which can be regarded as being monocrystalline is one inwhich a silicon film formed by the plasma CVD method or the reducedpressure thermal CVD method is used as the starting film, and the filmcontains carbon and nitrogen at a concentration of between 1×10¹⁶ and5×10¹⁸ cm⁻³ and oxygen at a concentration of between 1×10¹⁷ and 5×10¹⁹cm⁻³. Further, lattice defects are present in principle, and thereforehydrogen is contained at a concentration of between 1×10¹⁷ and 5×10²⁰cm⁻³ in order to neutralize unpaired bonds in the silicon. In otherwords a characteristic of the region which can be regarded as beingmonocrystalline is that although it has point defects it does not havelinear defects or surface defects. It should be noted that theconcentrations of elements which are contained is defined as being theminimum value as measured by SIMS (secondary ion mass spectroscopy).

The abovementioned region which can be regarded as being monocrystallineis different from a general monocrystalline wafer. This results from itsbeing a thin-film semiconductor with a thickness of betweenapproximately 200 and 2000 Å which is formed by the CVD method.

The construction of another invention is

-   -   a semiconductor device which employs a thin-film silicon        semiconductor which is formed on a substrate which has an        insulating surface, wherein    -   the abovementioned thin-film silicon semiconductor has a region        which can be regarded as being effectively monocrystalline,    -   the abovementioned region constitutes at least part of an active        layer,    -   and the abovementioned region contains carbon and nitrogen atoms        at a concentration of between 1×10¹⁶ cm⁻³ and 5×10¹⁸ cm⁻³, and        oxygen at a concentration of between 1×10¹⁷ cm⁻³ and 5×10¹⁹        cm⁻³.

The construction of another invention is

-   -   a semiconductor device which employs a thin-film silicon        semiconductor which is formed on a substrate which has an        insulating surface, wherein    -   the abovementioned thin-film silicon semiconductor has a region        which can be regarded as being effectively monocrystalline,    -   the abovementioned region constitutes at least part of an active        layer,    -   and the abovementioned region contains hydrogen atoms, which        neutralize the unpaired bonds in the silicon, at a concentration        of between 1×10¹⁷ cm⁻³ and 5×10²⁰ cm⁻³.

The construction of another invention is

-   -   a semiconductor device which employs a thin-film silicon        semiconductor which is formed on a substrate which has an        insulating surface, wherein    -   the abovementioned thin-film silicon semiconductor has a region        which can be regarded as being effectively monocrystalline,    -   the abovementioned region constitutes at least part of an active        layer,    -   the abovementioned region contains carbon and nitrogen atoms at        a concentration of between 1×10¹⁶ cm⁻³ and 5×10¹⁸ cm⁻³, oxygen        atoms at a concentration of between 1×10¹⁷ cm⁻³ and 5×10¹⁹ cm⁻³,        and hydrogen atoms, which neutralize the unpaired bonds in the        silicon, at a concentration of between 1×10¹⁷ cm⁻³ and 5×10²⁰        cm⁻³,    -   and the thickness of the abovementioned thin-film silicon        semiconductor is on average between 200 and 2000 Å.

The construction of another invention is

-   -   a semiconductor device which employs a thin-film silicon        semiconductor which is formed on a substrate which has an        insulating surface, wherein    -   a region of the abovementioned thin-film silicon semiconductor        which has a crystalline construction which can be regarded as        being effectively monocrystalline constitutes at least a        channel-forming region,    -   and the abovementioned channel-forming region contains carbon        and nitrogen atoms at a concentration of between 1×10¹⁶ cm⁻³ and        5×10¹⁸ cm⁻³, oxygen atoms at a concentration of between 1×10¹⁷        cm⁻³ and 5×10¹⁹ cm⁻³, and hydrogen atoms, which neutralize the        unpaired bonds in the silicon, at a concentration of between        1×10 ^(17 cm) ⁻³ and 5×10²⁰ cm⁻³.

The construction of another invention is

-   -   a method of fabricating a semiconductor device, which method        comprises a step in which a region which can be regarded as        being monocrystalline is formed by irradiating with laser light        a silicon thin film which is formed on a substrate which has an        insulating surface,    -   wherein the abovementioned laser light irradiation is performed        in a state in which the sample has been heated to a temperature        of between 450° C. and 750° C.

The construction of another invention is

-   -   a semiconductor device which has a construction in which a        plurality of thin-film transistors are connected in parallel,    -   and each of the abovementioned plurality of thin-film        transistors has a construction in which there are effectively no        crystal grain boundaries within the channel-forming region.

The construction of another invention is

-   -   a semiconductor device which has a construction in which a        plurality of thin-film transistors are connected in parallel,    -   each of the abovementioned plurality of thin-film transistors        has a construction in which there are effectively no crystal        grain boundaries within the channel-forming region,    -   and the abovementioned channel-forming region contains carbon        and nitrogen atoms at a concentration of between 1×10¹⁶ cm⁻³ and        5×10¹⁸ cm⁻³, oxygen atoms at a concentration of between 1×10¹⁷        cm⁻³ and 5×10¹⁹ cm⁻³, and hydrogen atoms, which neutralize the        unpaired bonds in the silicon, at a concentration of between        1×10¹⁷ cm⁻³ and 5×10²⁰ cm⁻³.

The construction of another invention is

-   -   a semiconductor device which has a construction in which a        plurality of thin-film transistors are connected in parallel,    -   each of the abovementioned plurality of thin-film transistors        has a construction in which there are effectively no crystal        grain boundaries within the channel-forming region,    -   the abovementioned channel-forming region has a thickness of        between 200 and 2000 Å,    -   and the abovementioned channel-forming region contains carbon        and nitrogen atoms at a concentration of between 1×10¹⁶ cm⁻³ and        5×10¹⁸ cm⁻³, oxygen atoms at a concentration of between 1×10¹⁷        cm⁻³ and 5×10¹⁹ cm⁻³, and hydrogen atoms, which neutralize the        unpaired bonds in the silicon, at a concentration of between        1×10¹⁷ cm⁻³ and 5×10²⁰ cm⁻³.

The construction of another invention is

-   -   a semiconductor device which has a construction in which a        plurality of thin-film transistors are connected in parallel,    -   and each of the abovementioned plurality of thin-film        transistors is constructed from a thin-film silicon        semiconductor in which the channel-forming region can be        regarded as being effectively monocrystalline.

By constructing a thin-film transistor using, as the active layer, aregion of a thin-film semiconductor transistor which region can beregarded as being monocrystalline, it is possible to obtain a thin-filmtransistor which has high voltage resistance and in which there is novariation or degradation of characteristics.

Further, by adopting a construction in which there are connected inparallel a plurality of thin-film transistors which are constructedusing, as the active region, a region of a thin-film siliconsemiconductor which region can be regarded as being monocrystalline, itis possible to obtain a construction with which it is possible to allowa large current to flow. With such constructions it is possible toobtain effects which are effectively the same as those obtained byincreasing the channel width. By adopting this construction it ispossible to obtain the same characteristics as with a transistor whichis formed using a semiconductor which can be regarded as beingmonocrystalline, and it is possible to obtain a large mobility, a largevoltage resistance and stabilized characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) to 1(D) show the fabrication steps of an embodiment of athin-film transistor.

FIG. 2(A) to 2(D) show the fabrication steps of an embodiment of athin-film transistor.

FIG. 3 shows the construction of an embodiment of a thin-filmtransistor.

FIG. 4 shows the relationship between the energy density of laser lightirradiation and the Raman intensity for cases in which the temperatureto which the sample is heated varies.

FIG. 5 shows the relationship between the energy density of laser lightirradiation and the full-width at half magnitude of the Raman spectrumfor cases in which the temperature to which the sample is heated varies.

FIG. 6 shows an example of a liquid crystal electrooptical device whichis integrated on a single substrate.

FIG. 7 shows an example of a Raman spectrum.

FIG. 8 shows the construction of an embodiment of a thin-filmtransistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

The present embodiment employs a glass substrate (Corning 7059) as thesubstrate, and is an example in which a thin-film transistor isfabricated at a temperature which is lower than the distortiontemperature of the glass substrate. The distortion temperature ofCorning 7059 glass is 593° C., and therefore heat treatments attemperatures above this temperature are not preferable since they giverise to shrinkage and deformation of the glass substrate. In particular,if use is made of a glass substrate which has a large surface area, foruse in a particularly large liquid crystal display device, shrinkage anddeformation of the glass substrate have significant effects.

Thus the thin-film transistor shown in the present embodiment ischaracterized in that the effects of heat on the substrate are greatlyreduced by the maximum temperature in the heat treatment step being 600°C. or less, and preferably 550° C. or less.

FIG. 1 shows the fabrication steps for the thin-film transistor given inthe present embodiment. Firstly, a silicon oxide film 102 is formed bythe sputtering method to a thickness of 3000 Å on a quartz substrate 101as a base film. An amorphous silicon film 103 is then formed by theplasma CVD method or the low-pressure thermal CVD method to a thicknessof 500 Å. (FIG. 1 (A))

After forming the amorphous silicon film 103, heat treatment isperformed at a temperature of 600° C., thereby crystallizing theamorphous silicon film 103. Laser light is then irradiated, crystalgrowth is effected centered on the region indicated by 104, and acrystalline region 106 which can be regarded as being monocrystalline isobtained. The laser light irradiation is performed while the sample orsurface being irradiated is heated to a temperature of 600° C. The laserlight irradiation is performed in the region indicated by 104 in FIG. 1,and at this time crystal growth proceeds outwards from the regionindicated by 104. (FIG. 1 (A))

In the abovementioned step, it is advantageous to introduce into theamorphous silicon film a metal element which promotes crystallization.In this way it is possible to form a region which can be regarded asbeing monocrystalline over a larger surface area.

Having obtained the region 106 which can be regarded as beingmonocrystalline, the active layer of the thin-film transistor is formedby patterning, using this region. Most preferably the whole of theactive layer is formed within the region which can be regarded as beingmonocrystalline. However, depending on the size of the active layer,there are cases in which the resulting monocrystalline region isrelatively small, and it is difficult to construct the whole of theactive layer. In these cases it is possible to arrange that there are nocrystal grain boundaries within the channel-forming region by making thechannel-forming region the region which can be regarded as beingmonocrystalline.

Having formed the active layer, a silicon oxide film 107 is formed as agate insulating film to a thickness of 1000 Å using the plasma CVDmethod A film whose main component is aluminum, containing 0.2%scandium, is then formed to a thickness of 6000 Å. A gate electrode 108is then obtained by patterning the film whose main component isaluminum.

An oxide layer 109 is then formed by performing anodic oxidation usingthe gate electrode 108 as the anode, in an ethylene glycol solutioncontaining 10% tartaric acid. The thickness of the oxide layer 109 isapproximately 2000 Å. Due to the presence of the oxide layer it ispossible to form offset gate regions in the subsequent step in whichimpurity ions are injected.

Impurity ions, phosphorus ions for an N-channel type thin-filmtransistor, or boron ions for a P-channel type thin-film transistor, arethen injected into the active layer. In this step, the gate electrode108 and the oxide layer 109 at the periphery thereof act as a mask, andimpurity ions are injected into the regions indicated by 110 and 114.The region 110 in which impurity ions have been injected then forms thesource region, and the region 114 forms the drain region. Further, theoxide layer 109 at the periphery of the electrode 108 acts as a mask,and offset gate regions 111 and 113 are formed simultaneously. Further,a channel-forming region 112 is also formed in a self-aligning manner.(FIG. 1 (C))

After completion of the impurity ion injection step, laser light isirradiated, thereby annealing the active layer which was damaged by theinjection of the impurity ions, and activating the injected impurities.This step may also be performed by irradiating with strong light such asinfrared light.

Further, a silicon oxide film 115 is formed to a thickness of 7000 Å bythe plasma CVD method as a layer insulating film. After passing througha hole-opening step, a source electrode 116 and a drain electrode 117are formed. Further, by performing heat treatment in a hydrogenatmosphere at 350° C. the thin-film transistor is completed. (FIG. 1(D))

In the thin-film transistor shown in the present embodiment, the activelayer is constructed from a region which has a construction which can beregarded as being monocrystalline, and it is therefore possible to solvethe problems of low voltage resistance resulting from crystal grainboundaries, and the problem of a large leak current.

Embodiment 2

The present embodiment indicates an example in which a thin-filmtransistor is constructed using a crystalline region which can beregarded as being monocrystalline, said region being formed byintroducing into an amorphous silicon film a metal element whichpromotes crystallization.

FIG. 2 shows the fabrication steps in the present embodiment. Firstly, asilicon oxide film 102 is formed to a thickness of 3000 Å by thesputtering method on a glass substrate 101 as a base film. An amorphoussilicon film 103 is then formed to a thickness of 1000 Å by the plasmaCVD method or the reduced-pressure thermal CVD method. An extremely thinoxide film (not shown) is then formed on the surface of the amorphoussilicon film by the UV oxidation method. This oxide film is intended toimprove solution wetting in the subsequent solution application step.The UV oxidation step which is performed here involves irradiation withUV light in an oxidizing atmosphere, thereby forming an extremely thinoxide film on the surface of the surface which is being irradiated.

A nickel acetate solution is then coated by the spin coating method ontothe surface of the amorphous silicon film 103 on which is formed theextremely thin oxide film, thereby forming a film 100 containing nickel.Due to the presence of the film 100, the nickel element is disposed incontact with the amorphous silicon film via the extremely thin oxidefilm.

In this state the amorphous silicon film 103 is transformed into acrystalline silicon film by subjecting it to heat treatment at 550° C.for 4 hours. Here, since nickel, which is a metal element which promotescrystallization, has been introduced, it is possible to obtain acrystalline silicon film by heat treating at 550° C. for approximately 4hours.

Having obtained the silicon film 103 which has been transformed into acrystalline silicon film by heat treatment, crystal growth is effectedfrom the region indicated by 104 in FIG. 2 by irradiating with laserlight. In the present embodiment, nickel, which is a metal element whichpromotes crystallization, has been introduced, and it is thereforepossible to obtain a region which can be regarded as beingmonocrystalline, as indicated by 106, simply.

Having thus obtained the region 106 which can be regarded as beingmonocrystalline, as indicated in FIG. 2 (B), the active layer of thethin-film transistor is formed using this region. Further, the filmcontaining nickel is removed before or after formation of the activelayer.

Having formed the active layer, a gate insulating layer 107 isconstructed from a silicon oxide film, and furthermore a gate electrode108 whose main component is aluminum, and an oxide layer 109 at theperiphery thereof are formed. These fabrication steps are the same asindicated in embodiment 1.

Having obtained the state shown in FIG. 2 (C) in this way, impurity ionsare injected, and a source region 110 and a drain region 114 are formed.In this step, offset gate regions 111 and 113, and also achannel-forming region 112 are formed in a self-aligning manner.

Furthermore, by irradiating with laser light, damage caused when theimpurity ions were injected is annealed, and impurity ions which havebeen injected are activated.

A silicon oxide film 115 is then formed as a layer insulating film bythe plasma CVD method, and after passing through a hole-opening step, asource electrode 116 and a drain electrode 117 are formed. Finally, byheat treating for 1 hour in a hydrogen atmosphere at 350° C., thethin-film transistor shown in FIG. 2 (D) is completed.

Embodiment 3

The present embodiment relates to a construction in which the inventionsdisclosed in the present specification are employed in a thin-filmtransistor which is required to handle large currents. For example, inthe peripheral circuitry of an active matrix-type liquid crystal displaydevice, a buffer amplifier (a power conversion circuit which has a lowoutput impedance) through which large currents can flow is required inorder to drive the many thousands of pixel transistors which areprovided. For cases in which not only the display region but also theperipheral circuitry region is to be integrated on a single substrate,there is the need to construct the buffer amplifier using thin-filmtransistors.

In order to construct a thin-film transistor which can be used in such abuffer amplifier, it is necessary for the channel-forming region of thethin-film transistor to have a width of several tens of micrometers ormore. However, if crystalline silicon thin films which have a generalmulticrystalline or microcrystalline construction are used, the voltageresistance is low, and there are problems in that it is difficult toconstruct the required buffer amplifier. Further, when high speedoperations are performed, there is the problem that variations and driftof the characteristics are liable to occur. This is due to the fact thatthe threshold value in each transistor varies, and characteristics arelikely to be degraded. Further, there are serious problems of heatgeneration, and there are also problems in that the characteristics aredegraded by the effects of heat generation. The main reason for theseproblems is that crystal grain boundaries exist within the active layer(in particular within the channel-forming region).

Thus the present embodiment provides a construction in which there areconnected, in parallel a plurality of thin-film transistors in each ofwhich the channel-forming region is constructed using a region which canbe regarded as being monocrystalline, and with which it is possible tohandle currents of a size similar to thin-film transistors which have anequivalently large channel width.

FIG. 3 shows the construction of a thin-film transistor indicated in thepresent embodiment. The construction given in the present embodimentshows a construction in which three thin-film transistors are connectedin parallel. The construction shown in FIG. 3 is a construction in whichthe active layer forming the channel-forming region and peripherythereof in each thin-film transistor is constructed from a siliconsemiconductor thin-film which can be regarded as being monocrystalline.

In FIG. 3, the regions indicated by 106 are regions which can beregarded as being monocrystalline. The regions which can be regarded asbeing monocrystalline, indicated by 106, include part of thechannel-forming region and source/drain regions. It is thereforepossible to construct not only the channel-forming region, but also theinterface between the source region and the channel-forming region andthe vicinity thereof, and the interface between the drain region and thechannel-forming region and the vicinity thereof such that they can beregarded as being monocrystalline.

When such a construction is adopted, it is possible to solve theproblems which arise due to the presence of crystal grain boundaries. Inother words, it is possible to solve the problem of low voltageresistance, the problem of degradation of characteristics and theproblem of variations in threshold values. Further, since it is possibleto reduce the number of carriers which migrate between the source anddrain via the crystal grain boundaries it is possible to reduce the offcurrent.

The cross section through A-A1 of the construction shown in FIG. 3corresponds to FIG. 1 (D). In other words, the construction shown inFIG. 3 has a construction in which three thin-film transistors shown inFIG. 1 (D) are connected in parallel. Each transistor has a common gateelectrode, and the source electrode and drain electrode are wired incommon by contacts 305 and 306.

If the construction shown in the present embodiment is adopted, it ispossible to perform operations equivalent to using a thin-filmtransistor which has a channel width of 60 μm, even though the channelwidth of each thin-film transistor is 20 μm, by connecting threethin-film transistors in parallel.

The present embodiment shows an example in which three thin-filmtransistors are connected in parallel. However, the number of thin-filmtransistors which are connected in parallel can be selected asnecessary.

By adopting a construction such as shown in the present embodiment it ispossible to obtain a thin-film transistor which has characteristicswhich are similar to a thin-film transistor which employs asemiconductor which can be regarded as being monocrystalline, and whichcan handle large currents. It is therefore possible to perform highspeed operations, and it is furthermore possible to achieve aconstruction with which there is no degradation or variation ofcharacteristics.

The construction shown in the present embodiment can be said to be idealfor circuits which require a large current to flow, for example buffercircuits which are provided in the peripheral circuitry of activematrix-type liquid display devices.

Embodiment 4

FIG. 6 shows the architecture of a high-accuracy active matrix liquidcrystal display system which employs the invention disclosed in thepresent specification. The example shown in FIG. 6 is an example whichhas been made small, lightweight and thin by fixing semiconductor chipswhich are normally attached to the main board of a computer onto atleast one of the substrates of a liquid crystal display which has aconstruction in which liquid crystal is sandwiched between a pair ofsubstrates.

A description of FIG. 6 will now be given. The substrate 15 is asubstrate of a liquid crystal display, and an active matrix circuit 14which is formed from a number of pixels which are provided with a TFT11, a pixel electrode 12 and an auxiliary capacitor 13, an Xdecoder/driver, a Y decoder/driver and an XY branching circuit areformed on the substrate 15 using TFTs. In order to drive the activematrix circuit, a buffer circuit which has a low output impedance mustbe provided in the peripheral circuit, and this buffer circuit isadvantageously constructed using a circuit such as shown in FIG. 3.

Other chips are also attached to the substrate 15. These chips areconnected to the circuitry on the substrate 15 by a means such as thewire bonding method or the COG (chip on glass) method. In FIG. 6, thechips which are attached in this way are the correction memory, thememory, the CPU and the input board, but various other chips may also beattached.

In FIG. 6, the input board is a circuit which reads signals which havebeen input from outside, and converts them into image signals. Thecorrection memory is a memory which is unique to the active matrix paneland is used to correct input signals and the like in order to match themto the characteristics of the panel. In particular, the correctionmemory uses nonvolatile memory for information unique to each pixel, andcorrects them individually. In other words, if there is a point defectsin a pixel of an electrooptical device then signals are sent to pixelsaround this point to match with it, thereby covering the point defectsuch that the defect is not noticeable. Further, if a pixel is darkerthan the surrounding pixels then a larger signal is sent to that pixelsuch that it has the same brightness as the surrounding pixels. Thedefect information for pixels is different for each panel, and thereforethe information stored in the correction memory is different for eachpanel.

The functions of the CPU and memory are the same as for a normalcomputer, and in particular memory is provided as RAM for the imagememory corresponding to each pixel. All of these chips are CMOS type.

It is further possible to increase the thin film of the system byconstructing at least part of the required integrated circuits using theinventions disclosed in the present specification.

In this way, forming even the CPU and memory on the liquid crystaldisplay substrate, and constructing an electronic device such as asimple personal computer on a single substrate is extremely advantageousin reducing the size of liquid crystal display systems and wideningtheir scope of application.

It is possible to use the thin-film transistors fabricated using theinventions disclosed in the present specification for circuits which arerequired in systemized liquid crystal displays. In particular, it isextremely advantageous to use thin-film transistors fabricated usingregions which can be regarded as being monocrystalline in analog buffercircuits or other necessary circuits.

Embodiment 5

The present embodiment relates to a construction in which threethin-film transistors are connected in parallel as shown in FIG. 8. InFIG. 8, 804 indicates a common active layer, and 803 is a region whichcan be regarded as being monocrystalline and which is formed in theactive layer. In FIG. 8, three regions which can be regarded as beingmonocrystalline are shown, and the channel-forming regions of eachthin-film transistor are formed in the three regions which can beregarded as being monocrystalline.

801 is a common gate electrode and gate wiring. 805 is a common sourceelectrode and source wiring. 806 is a common drain electrode and drainwiring. Further, 802 indicates contact portions between the source/drainelectrodes and the source/drain regions.

By employing the inventions disclosed in the present specification it ispossible to obtain a thin-film transistor which is not affected bycrystal grain boundaries. It is further possible to obtain a thin-filmtransistor which has high voltage resistance and with which there is novariation in characteristics, and with which it is possible to handlelarge currents. Further, since it is possible for the operation of thethin-film transistor not to be affected by crystal grain boundaries, itis possible for the off current to have a small characteristic.

1. A display device, comprising: a plurality of pixel electrodes formedon a substrate; a plurality of first thin film transistors, which areconnected to corresponding pixel electrodes among the plurality of pixelelectrodes and respectively supply the connected corresponding pixelelectrode with a display signal; and a plurality of second thin filmtransistors, which configure a driving circuit for driving the pluralityof first thin film transistors; wherein, the plurality of second thinfilm transistors each has a plurality of channel areas formed in asemiconductor layer subjected to laser annealing respectively, and theplurality of channel areas of each respective second thin filmtransistor are electrically connected in parallel to each other andarranged separately.
 2. The display device according to claim 1, whereinthe plurality of channel areas are separated in a direction of thechannel width.
 3. The display device according to claim 1, wherein thelaser annealing is performed to crystallize an amorphous semiconductorlayer in order to obtain a crystalline semiconductor layer.
 4. A displaydevice, comprising: a plurality of pixel electrodes formed on asubstrate; a plurality of first thin film transistors, which areconnected to corresponding pixel electrodes among the plurality of pixelelectrodes and respectively supply the connected corresponding pixelelectrode with a display signal; and a plurality of second thin filmtransistors, which configure a driving circuit for driving the pluralityof first thin film transistors; wherein some of the plurality of secondthin film transistors each has a plurality of channel areas formed in asemiconductor layer subjected to laser annealing respectively, and theplurality of channel areas of each respective second thin filmtransistor are electrically connected in parallel to each other andarranged separately.
 5. The display device according to claim 4, whereinthe plurality of channel areas are separated in a direction of thechannel width.
 6. The display device according to claim 4, wherein thelaser annealing is performed to crystallize an amorphous semiconductorlayer in order to obtain a crystalline semiconductor layer.
 7. A displaydevice, comprising: a plurality of pixel electrodes formed on asubstrate; a plurality of first thin film transistors, which areconnected to corresponding pixel electrodes among the plurality of pixelelectrodes and respectively supply the connected corresponding pixelelectrode with a display signal; and a plurality of second thin filmtransistors, which configure a driving circuit for driving the pluralityof first thin film transistors; wherein, at least one of the pluralityof second thin film transistors has a plurality of channel areas formedin a semiconductor layer subjected to laser annealing respectively, andthe plurality of channel areas of each respective second thin filmtransistor are electrically connected in parallel to each other andarranged separately.
 8. The display device according to claim 7, whereinthe plurality of channel areas are separated in a direction of thechannel width.
 9. The display device according to claim 7, wherein thelaser annealing is performed to crystallize an amorphous semiconductorlayer in order to obtain a crystalline semiconductor layer.
 10. Adisplay device, comprising: a plurality of pixel electrodes formed overa substrate; a plurality of first thin film transistors, which areconnected to corresponding pixel electrodes among the plurality of pixelelectrodes and respectively supply the connected corresponding pixelelectrode with a display signal; and a plurality of second thin filmtransistors, which configure a driving circuit for driving the pluralityof first thin film transistors; wherein, the plurality of second thinfilm transistors each has a plurality of channel areas formed in asemiconductor layer subjected to laser annealing respectively, and theplurality of channel areas of each respective second thin filmtransistor are electrically connected in parallel to each other andarranged separately.
 11. The display device according to claim 10,wherein the plurality of channel areas are separated in a direction ofthe channel width.
 12. The display device according to claim 10, whereinthe laser annealing is performed to crystallize an amorphoussemiconductor layer in order to obtain a crystalline semiconductorlayer.
 13. A display device, comprising: a plurality of pixel electrodesformed over a substrate; a plurality of first thin film transistors,which are connected to corresponding pixel electrodes among theplurality of pixel electrodes and respectively supply the connectedcorresponding pixel electrode with a display signal; and a plurality ofsecond thin film transistors, which configure a driving circuit fordriving the plurality of first thin film transistors; wherein, some ofthe plurality of second thin film transistors each has a plurality ofchannel areas formed in a semiconductor layer subjected to laserannealing respectively, and the plurality of channel areas of eachrespective second thin film transistor are electrically connected inparallel to each other and arranged separately.
 14. The display deviceaccording to claim 13, wherein the plurality of channel areas areseparated in a direction of the channel width.
 15. The display deviceaccording to claim 13, wherein the laser annealing is performed tocrystallize an amorphous semiconductor layer in order to obtain acrystalline semiconductor layer.
 16. A display device, comprising: aplurality of pixel electrodes formed over a substrate; a plurality offirst thin film transistors, which are connected to corresponding pixelelectrodes among the plurality of pixel electrodes and respectivelysupply the connected corresponding pixel electrode with a displaysignal; and a plurality of second thin film transistors, which configurea driving circuit for driving the plurality of first thin filmtransistors; wherein, at least one of the plurality of second thin filmtransistors has a plurality of channel areas formed in a semiconductorlayer subjected to laser annealing respectively, and the plurality ofchannel areas of each respective second thin film transistor areelectrically connected in parallel to each other and arrangedseparately.
 17. The display device according to claim 16, wherein theplurality of channel areas are separated in a direction of the channelwidth.
 18. The display device according to claim 16, wherein the laserannealing is performed to crystallize an amorphous semiconductor layerin order to obtain a crystalline semiconductor layer.